As a known cap structure (sealing device) used for a container with a closure, there is a structure which is provided with an inner plug and an upper closure, and in which a cylindrical part fitted with an opening part of a bottle and a separation part continuously provided on an inner side thereof via a score are provided in the inner plug (refer to FIGS. 1 and 2 in Patent Literature 1).
In this cap structure, the upper closure is preliminarily engaged (screwed) with the separation part of the inner plug through, for example, a forward thread or reverse thread. As a result, a movement difference can be generated between the separation part of the inner plug and the upper closure by screwing backward so as to remove the upper closure from the opening part (forward thread) of the bottle when opening the plug, whereby the separation part is separated from the cylindrical part of the inner plug and the separation part is held by the upper closure. According to this cap structure, the upper closure can be easily opened only by rotating the upper closure so as to be loosened, and the separation part separated by opening the plug is held on an inner cylinder on an upper end of the upper closure, and thus the separation part after separation can be made to function as a seal, whereby the separation part does not become so-called rubbish.
Incidentally, in the cap structure in Patent Literature 1, the structure is such that, when opening the plug, a movement difference is generated between the separation part and the upper closure by rotation of the separation part of the inner plug through the reverse thread or forward thread with respect to the upper closure, and thus there is a problem in which the separation part is not separated until the movement difference reaches a certain level or more as long as a cutting member having an edge is not provided in a supplementary manner. Particularly, in the case of the forward thread, the movement difference becomes less likely to be generated, and even in the case of the reverse thread, a feeling of snap-opening of the plug (clear opening feeling) is less likely to be generated.